Avermectins and ivermectin are excellent pesticides and veterinary drugs, which have got a wide range of use in the last three decades. Among these, avermectins belong to a class of macrolide derivatives having insect-resistant activity produced from the fermentation of Streptomyces avermitilis. Avermectins isolated from the fermentation products of Streptomyces avermitilis are composed of a series of structurally similar compounds existed in pairs, including component A and component B, wherein the activity of component B is higher than that of component A. Structurally, component A has a methoxy at C5 position of the macrocyclic lactone ring of avermectins, whereas component B has a hydroxyl group at the same position. As far as component B of avermectins is concerned, it includes two compounds existed in pairs, which are respectively called B1a and B1b and have the following structures:

Commercially available pesticide of avermectins typically contains active ingredient of avermectins mainly composed of avermectins B1a and B1b. Ivermectin is obtained from structural modification of avermectin. But after nearly three decades of use, it will be phased out in the near future because of drug resistance problems. Milbemycin belongs to a new generation of avermectin analogues produced from the fermentation of Streptomyces hygroscopicus. The insect-resistant activity of milbemycin is stronger than those of avermectins and ivermectin, but the toxicity of milbemycin is greatly lower than both of them.
U.S. Pat. No. 4,134,973 discloses a carbohydrate derivative of milbemycin and 13-hydroxyl milbemycin and methods for their preparation through a series of chemical reactions, and confirms that these carbohydrate derivatives have insect-resistant activity. However, these chemical reactions require a series of complex processes and require special reaction raw materials, and will produce a series of by-products because of specificity that is not strong enough, which will bring about environmental problems and also increase the cost of production.
Therefore, there exists the need for an alternative method of producing avermectin or analogues thereof by biosynthetic methods.